1. Field of the Invention
The present invention relates to a method of driving a liquid crystal panel and a liquid crystal display apparatus for displaying high-quality images when multicolor display or full-color display is performed or when the liquid crystal panel for displaying an image is increased in size.
2. Description of the Related Art
A conventional active-matrix liquid crystal display apparatus using a thin-film transistor (hereinafter, referred to as “TFT”) includes a liquid crystal panel comprising a plurality of pixels arranged in a matrix form, and a liquid crystal driving portion for supplying electric signals to the liquid crystal panel. The pixels each have a structure such that liquid crystal is sandwiched between a pixel electrode and a counter electrode. In addition to the plurality of pixels, the liquid crystal panel includes a plurality of scanning lines, a plurality of data lines and a plurality of TFTs. The pixel electrode in each pixel is connected to one of the data lines through one of TFTs. The counter electrodes of all the pixels are interconnected to one another to form one common electrode. The liquid crystal driving portion includes a gate driver for supplying electric signals to the scanning lines, a source driver for supplying electric signals to the data lines, and the common electrode.
FIG. 10 is a block diagram showing the electric structure of the source driver 1. The source driver 1 includes an input latch circuit 2, a shift register 3, a sampling memory 4, a hold memory 5, a D/A converter 6, a gradation-voltage-generating circuit 7 and an output circuit 8. The source driver 1 is supplied with the image data representative of the image to be displayed. The image data comprises data for expressing the brightness, chroma and hue of each image element forming the image. Each image element of the image corresponds to a set of three pixels having red, blue and green color filters, respectively, in the liquid crystal panel. Therefore, each image element data comprises three kinds of gradation components, namely, so-called R (red), G (green) and B (blue) components, and the gradation components represent 64 levels of gradations.
First, the three kinds of gradation components in each image element data are successively supplied to the input latch circuit 2 to be latched. Based on a synchronizing signal SPI supplied from a control circuit disposed outside the source driver 1 through the shift register 3 that operates in response to a clock signal CK, the sampling memory 4 samples the image data latched by the input latch circuit 2. Consequently, a part of the image data which part is associated with an electric signal to be supplied from the source driver 1 to the liquid crystal panel within a single horizontal period 1H, that is, a plurality of gradation components that decide the gradations of a plurality of pixels constituting one of the rows in the liquid crystal panel are stored in the sampling memory 4. The plurality of gradation components are transferred from the sampling memory 4 to the hold memory 5 in synchronism with a synchronizing signal LS of the horizontal period of the liquid crystal panel.
The hold memory 5 latches the plurality of gradation components being transferred, and supplies the plurality of gradation components to the D/A converter 6. The gradation-voltage-generating circuit 7 divides the difference between predetermined two reference voltages Vref1 and Vref2, decides 64 kinds of gradation voltages and supplies the gradation voltages to the D/A converter. The gradation voltages each correspond to one of the 64 levels of gradations that the pixels can take. The D/A converter 6 selects from among the 64 kinds of gradation voltages the gradation voltages that correspond to the gradations shown by the plurality of gradation components being supplied, and supplies the selected gradation voltages to the output circuit 8. The output circuit 8 impedance-converts the selected gradation voltages and charges or discharges the source lines of the liquid crystal panel in accordance with the impedance-converted gradation voltages. Consequently, to the source lines of the liquid crystal panel, electric signals of voltages based on the image data are supplied as so-called data signals.
In each of the pixels, since the pixel electrode and the counter electrode act as electrodes of a capacitor, a capacitance called, for example, parasitic capacitance is present. That is, data signals in accordance with the voltages to be held by the pixels are supplied from the source driver to the data lines and the states of the TFTs are changed, whereby the voltages can be written into the pixels so as to be held by the pixels.
For example, with respect to one of all the TFTs, when the voltage of an electric signal, namely, a so-called scanning signal, supplied from the gate driver to the scanning line to which the gate terminal of the TFT is connected becomes positive, a positive voltage is applied to the gate terminal, so that the one of the TFTs changes to a so-called ON state. Consequently, the pixel including the pixel electrode to which the one of the TFTs is connected is charged by the voltage applied to the data line to which the one of the TFTs is connected. When the voltage of the scanning signal becomes negative, a negative voltage is applied to the gate terminal, so that the one of the TFTs changes to so-called OFF state. Consequently, the voltage between the pixel electrode and the counter electrode in the pixel is maintained at the voltage applied between the pixel electrode and the counter electrode when the one of the TFTs changes to OFF state. As a result, the voltage to be held is written into the pixel. The transmittance of the liquid crystal layer in the pixel, that is, the gradation of the pixel is decided in accordance with the voltage held by the pixel. Therefore, by controlling the gradations of all the pixels in the liquid crystal panel by the voltages held by the pixels, an image is displayed on the liquid crystal panel.
The liquid crystal panel is reversely driven in order that the liquid crystal is not polarized. Reverse driving methods include a so-called dot reversal driving method and a so-called line reversal driving method. In the description that follows, it is assumed that the pixels of the liquid crystal panel are arranged in 6 rows and 5 columns.
First, the behavior of the liquid crystal display apparatus of the above-described structure when the liquid crystal display apparatus is driven by the line reversal driving method will be described. FIG. 11 shows a timing chart of a plurality of scanning signals 11a to 11f supplied from the gate driver in the liquid crystal display apparatus to six scanning lines. FIG. 12 shows a timing chart of one scanning signal 11 of the scanning signals 11a to 11f, one data signal 12 of a plurality of data signals supplied from the source driver 1 to five data lines, and a voltage 13 applied to the common electrode in the liquid crystal display apparatus. FIGS. 11 and 12 will be described together.
The scanning signals 11a to 11f are held at high level during a predetermined single horizontal period WH at intervals of a predetermined frame display period CH, and are held at low level during the remaining period. The timing where the plurality of scanning signals 11a to 11f are held at high level within a time period corresponding to one cycle of a horizontal synchronization cycle differs among the signals. Therefore, to all the pixels in the row of pixels on one of the scanning lines, the voltage to be held is written while the scanning signal supplied to the one of the scanning lines is held at a high level. The row as pixels on one of the scanning lines is a set of a plurality of pixels including pixel electrodes connected to the drain terminals of a plurality of TFTs whose gate terminals are connected to the one of the scanning lines.
The cycle of the alternating component of the voltage 13 applied to the common electrode equals the horizontal period WH. That is, when the line reversal driving method is used, the common electrode is AC-driven in a cycle the same as the horizontal period WH by a single 5-V power source. The alternating component of the data signal 12 changes in a predetermined cycle that is shorter than the horizontal period WH around the center of amplitude of the alternating component of the voltage 13 applied to the common electrode. The amplitude of the alternating component of the data signal 12 varies according to the gradation of the pixel. The alternating component of a data signal 12a of a case where the gradation of the pixel is maximum, that is, a case where the pixel represents black is opposite in polarity to the alternating component of a data signal 12b of a case where the gradation of the pixel is minimum, that is, a case where the pixel represents white. The amplitudes of the data signals 12a and 12b of the cases where the gradation of the pixel is maximum and where the gradation is minimum are both smaller than the amplitude of the alternating component of the voltage 13 applied to the common electrode.
The arrow 14 indicates the polarity of the current flowing through the pixel in order to write the voltage to be held into the pixel, that is, whether or not the voltage held by the data line is higher than the voltage held by the common electrode when the voltage to be held is written into the pixel. When the arrow 14 points upward, since the voltage of the data line is higher than the voltage of the common electrode, the polarity is positive. When the arrow 14 points downward, since the voltage of the data line is lower than the voltage of the common electrode, the polarity is negative. When the polarity is positive, the current flows from the data line through the pixel to the common electrode. When the polarity is negative, the current flows from the common electrode through the pixel to the data line.
FIG. 13A shows the polarities of the currents in all the pixels. The currents are for writing the voltages to be held into all the pixels in the liquid crystal panel in a given frame in case where the liquid crystal display apparatus is driven by the use of the line reversal driving method. FIG. 13B shows the polarities of the currents in all the pixels in a frame next to the frame of FIG. 13A in the above-mentioned case. The plural rectangles arranged in a matrix correspond to the pixels in the liquid crystal panel of 6 rows and 5 columns. The rows of the rectangles correspond to the rows of pixels. The columns of the rectangles correspond to the columns of pixels, that is, sets of all the pixels including the pixel electrodes connected to one given data line through the TFTs. When the polarity of the current flowing through a pixel is positive, “+” is drawn in the rectangle corresponding to the pixel. When the polarity is negative, “−” is drawn in the rectangle.
The polarity of the current flowing through one given pixel of the liquid crystal panel is reversed between the first frame and the next frame. In both of the first and the next frames, the polarities of the currents flowing through two adjoining pixels in one column are different from each other and the polarities of the currents flowing through all the pixels in one row are equal to one another. Consequently, the currents concentrate at the common electrode, so that a voltage drop is apt to occur at the common electrode. When a voltage drop occurs, it is impossible to correctly write the voltages to be held into the pixels, so that the display quality of the liquid crystal display apparatus is reduced.
A cause of display quality reduction of the liquid crystal display apparatus when the liquid crystal display apparatus is driven by the use of the line reversal driving method will be described in detail by the use of an equivalent circuit of the liquid crystal display apparatus of FIG. 14. In FIG. 14, it is assumed that the pixels of a liquid crystal panel 20 are arranged in 2 rows and 2 columns, and the common electrode is shown as a plurality of counter electrodes 22 successively connected by conductors 25 each having an internal resistance component rc.
For example, it is assumed that the voltages to be held are written into pixels 21a and 21b on the first scanning line 24a from the top by a positive-polarity current. In this case, the voltage of the scanning signal supplied to the first scanning line 24a based on an output 23a from the gate driver is a voltage capable of turning on the TFTs, whereas the voltage of the scanning signal supplied to the second scanning line from the top based on an output 23b from the gate driver is a voltage capable of turning off the TFTs. In the above-described case, the currents flowing into the pixels 21a and 21b of the row on the first scanning line 24a flow, as shown by the broken line 30, from data lines 26a and 26b through TFTs 27a and 27b and pixels 28a and 28b to parts 29 on the side of the common electrode.
As described above, when the polarities of the currents written into all the pixels of the row on one given scanning line are equal to one another, the directions of the currents flowing through all the pixels are equal to one another. Therefore, the currents flowing out of all the pixels concentrate at the common electrode, so that a voltage drop occurs due to the resistance components rc of the conductors 25 interposed between the counter electrodes 22 and the internal resistance Rc of the parts 29 on the side of the common electrode. Consequently, as shown in FIG. 15, the actual voltage Vα between the common electrode and the pixel electrode is lower than the difference Vβ between the voltage of the data signal and the voltage applied to the common electrode by the amount Vγ of the voltage drop. That is, the voltage actually held by the common electrode is closer to the voltage of the common electrode than a voltage to be intrinsically held by the pixel electrode by the voltage drop amount Vγ.
The voltage drop amount Vγ varies according to the voltage of the data signal. For example, the voltage drop amount Vγ is largest when all of the voltages of all the data signals supplied to the liquid crystal panel within the horizontal period WH are the highest pixel voltage of the pixel voltages of the 64 gradations. Moreover, for example, the voltage drop amount is smallest when all of the voltages of all the data signals are the lowest pixel voltage of the pixel voltages of the 64 gradations. The levels of all the data signals are decided in accordance with the gradation distribution of the image elements of one of the rows in the image represented by the image data and the gradation distributions of the image elements of the rows in the image frequently differ from one another. Therefore, the levels of all the data signals change at intervals of a horizontal period, that is, every time the row into which the voltage to be held is written is changed.
Consequently, when a sheet of image is displayed on the liquid crystal panel, so-called gradation nonuniformity is caused in the image. Further, when an image in which there is a black window against a halftone background is displayed on the liquid crystal display apparatus, the peripheral part of the black window in the background is whiter than the part other than the peripheral part in the background. Therefore, in the above-described case, so-called lateral shadowing becomes a problem. From the above, when the liquid crystal display apparatus is driven by the use of the line reversal driving method, the display quality of the liquid crystal display apparatus is reduced.
Hereinafter, the behavior of the liquid crystal display apparatus of the above-described structure when the liquid crystal apparatus is driven by the dot reversal driving method will be described. FIG. 16 shows a timing chart of a scanning signal 31, a data signal 32 and a voltage 33 applied to the common electrode in the liquid crystal display apparatus. The definitions of the signals 31, 32a, 32b and 33 and the definition of the arrow 34 are the same as the definitions of the signals 11, 12a, 12b and 13 and the definition of the arrow 14 of FIG. 12, respectively. The scanning signal 31 is the same as the scanning signal 11 of FIG. 12. The alternating component of the data signal 32 changes in a cycle shorter than the horizontal period WH. The voltage 33 applied to the common electrode is always held at the center of amplitude of the alternating component of the data signal 32. Therefore, the liquid crystal display apparatus is driven so that the voltage of the common electrode is always the same and that the voltages of all the pixel electrodes are symmetrical with respect to the voltage of the common electrode.
FIG. 17A shows the polarities of the currents in all the pixels which currents are for writing the voltages to be held into all the pixels in the liquid crystal panel in a given frame in a case where the liquid crystal display apparatus is driven by the use of the dot reversal driving method. FIG. 17B shows the polarities of the currents in all the pixels in a frame next to the frame of FIG. 17A in the above-mentioned case. The definitions of the rectangles, “+” and “−” of FIGS. 17A and 17B are the same as the definitions of the rectangles, “+” and “−” of FIGS. 13A and 13B.
The polarities of the currents flowing through the pixels of the liquid crystal panel differ between the first frame and the next frame. In both of the first and the next frames, the polarities of the currents flowing through two adjoining pixels in one column are different from each other and the polarities of the currents flowing through two adjoining pixels in one row are different from each other. Consequently, when the voltages are written into all the pixels in the row on one of the scanning lines, the directions of flow of the currents for writing the voltages into two adjoining pixels are opposite to each other, so that the currents flowing from the two adjoining pixels cancel each other out. Therefore, the voltage of the common electrode is stabilized, so that the voltage held by the pixel electrode does not vary.
Conventional liquid crystal display apparatuses of which liquid crystal panel is driven by the use of the dot reversal driving method include an active-matrix liquid crystal display apparatus of Japanese Publication for Laid-Open Patent Application Hei 5-341732 (1993). In this liquid crystal display apparatus, in accordance with the amplitude of the alternating component of the data signal, the voltage of the common electrode is regulated so as to be always the same as the center of voltage variation of the pixel electrode.
In a liquid crystal display apparatus using the dot reversal driving method, for example, the active-matrix liquid crystal display apparatus of JP-A 5-341732, the integrated circuit constituting the source driver requires a driving voltage approximately twice the driving voltage required by the integrated circuit constituting the source driver in the liquid crystal display apparatus using the line reversal driving method. Therefore, while a so-called low withstand process can be used for the latter integrated circuit, it is necessary to use an intermediate withstand process for the former integrated circuit. Therefore, the size of the integrated circuit of the liquid crystal display apparatus using the dot reversal driving method is larger than the size of the integrated circuit of the liquid crystal display apparatus using the line reversal driving method, and the number of masks necessary for manufacturing the former integrated circuit is greater than the number of masks necessary for manufacturing the latter integrated circuit. Consequently, the manufacturing process of the integrated circuit of the liquid crystal display apparatus using the dot reversal driving method is more complicated than the manufacturing process of the integrated circuit of the liquid crystal display apparatus using the line reversal driving method.
From these, the manufacturing cost of the integrated circuit of the liquid crystal display apparatus using the dot reversal driving method is higher than the manufacturing cost of the integrated circuit of the liquid crystal display apparatus using the line reversal driving method. Moreover, since the integrated circuit of the liquid crystal display apparatus using the dot reversal driving method employs the intermediate withstand process, it is necessary that the power circuit for supplying power for driving the integrated circuit withstand higher voltages than conventional power circuits. For this reason, it is necessary to newly develop a power circuit that withstands voltages of at least 10 V.
As described above, when the liquid crystal display apparatus of the above-described structure is driven by the use of the line reversal driving method, the display quality of the liquid crystal display apparatus is reduced due to shadowing and nonuniformity in brightness. When the liquid crystal display apparatus of the above-described structure is driven by the use of the dot reversal driving method, it is impossible to use the low withstand process for the driver in the liquid crystal driving portion, so that the manufacturing cost of the liquid crystal display apparatus increases.